Radio navigation signals are used to measure range between a transmitter antenna and receiver antenna. In many environments, the transmitted radio navigation signal is reflected from objects around the transmitter and/or receiver antennae and/or along the transmission path. In this type of environment, the resultant received radio navigation ranging signal is a combination of all of the radio navigation ranging signals including reflected signals and the desired direct signal. This combination of multiple radio navigation ranging signals, called multipath, corrupts the radio navigation ranging signals and therefore degrades the accuracy of the range measurement. Multipath is a significant error source in radio navigation systems.
Direct Sequence Spread Spectrum Code Division Multiple Access Radio Navigation A common radio navigation signal structure is Direct Sequence Spread Spectrum (DSSS) Code Division Multiple Access (CDMA). Radio navigation systems such as the Global Positioning System (GPS), the proposed Galileo system, and the Russian built GLONASS system all use a DSSS CDMA radio navigation signal. With the DSSS CDMA signal, the transmitted signal is continuous, not pulsed, and is spread by a digital spreading sequence within the transmitter during the generation of the radio navigation signal. A receiver, knowing the digital spreading sequence, can then despread the radio navigation signal if the same digital spreading sequence is applied to the incoming signal in time alignment with the transmitted radio navigation signal. That is, the digital spreading sequence applied to the radio navigation signal within the receiver must be applied at the same location in the received signal as it was applied to the signal in the transmitter. In terms of time, this means that the digital spreading sequence is applied in the receiver at the same time as it is applied within the transmitter corrected for the flight time.
The digital representation of the radio navigation signal is modulated with the digital spreading sequence. The radio navigation signal is received by the receive antenna then converted to a digital signal. A typical radio navigation receiver converts the signal from the radio frequency (RF) to an intermediate frequency (IF). The signal is then sampled with an Analog to Digital Converter (ADC) to provide the digital representation, or digital samples, of the radio navigation signal. There are numerous mathematical processes for applying a digital spreading sequence to a received signal such as correlation, convolution, match filtering, and Fast Fourier transformations (FFTs). For the purposes of describing the present invention, the term correlation is used as a generalization for all the mathematical process of applying the digital spreading sequence to the received radio navigation signal.
The digital spreading sequence must be applied to the received radio navigation signal at the transmit time corrected for the flight time to properly recover the radio navigation signal generated by the transmitter. However, in a ranging system the flight time is generally not known. Therefore, the receiver must search through the possible time offsets of the digital spreading sequence to find the precise time offset that provides the correct recovery of the original radio navigation signal generated within the transmitter. During this search process, the recovered radio navigation signal is only available when the digital spreading sequence is time aligned to within plus or minus one (1) element, or chip, of the digital spreading sequence. Outside of this one (1) chip plus or minus receiver time alignment, the resulting recovered signal is not the original radio navigation signal generated by the transmitter, but is instead noise. When within the one (1) chip plus or minus receiver time alignment, the power of the recovered radio navigation signal varies across the two-chip validity range with a known pattern. For the mathematical process of correlation, the power of the recovered radio navigation signal across this two-chip span is termed the auto correlation response function, or simply the correlation response function.
When correlating uncorrupted, full bandwidth, CMDA signals, the correlation response function in the time domain is substantially a triangle. The peak of the triangle, or the maximum value of the correlation response function, is interpreted by the receiver as a direct measurement of the time the receiver obtained the radio navigation ranging signal. The shape of the correlation response function is controlled by the filtering and sampling capabilities of the receiver and by the noise floor. Increased filtering rounds off the top of the triangle and decreases the width across the base. Limited sampling capability in the receiver and the overall noise floor push the correlation response function triangle shape down into the noise. None of these effects change the location of the maximum correlation response function power.
The effect of multipath on the correlation response function is to distort the response function, often involving moving the maximum of correlation response in time, and therefore creating an error in the resulting range measurement. Multipath distortion may also increase or decrease the amplitude of the correlation response function, increase or decrease the time span of the correlation response function, and/or change the shape of the correlation response function.
The optimal time delay measurement of the radio navigation ranging signal is the peak or maximum value of the correlation response function. Determining the exact peak power requires exact knowledge of the receive time of the incoming radio navigation ranging signal to allow placement of the correlation response function power detector in the exact maximum time delay location. To avoid this problem, a typical GPS receiver takes power measurements at half a chip early and half a chip late timing relative to the peak. The code tracking loop balances the power between these two time-delay locations on the correlation response function, thereby providing an estimate of the peak as half way between the early and late balanced power measurement in the correlation response function. The spacing of these correlation response function power measurements can vary from the +/− half chip spacing described above. Some receivers use a narrower spacing, for example +/−0.1 of a chip. However, the basic function of balancing the power between the early and late correlation response function power is the same. This technique in effect finds the centroid of the correlation response function and uses that as the estimate of the maximum correlation power, and hence it forms the basis of the range estimate.
When the correlation response is distorted with multipath, the estimated peak value of the correlation response is in error because of the distortion in power measurements along the correlation response function. When power measurements are distorted, the code tracking loop attempts to balance the power between two incorrect power estimates and therefore incorrectly identifies the multipath-free correlation peak.
For illustrative purposes, FIG. 1 shows the two correlation response functions for an ideal CDMA radio navigation signal of the prior art incorporating: a) the theoretical, non-filtered correlation response function 101 and b) the practical, filtered correlation response function of a typical navigation receiver 102. The center 104 of the received correlation response function is in the same time delay location as the centre 103 of the non-filtered theoretical correlation response function. FIG. 2 shows the same two correlation responses of FIG. 1; however the received signal in this case has the same direct signal plus a 0.5 chip delay multipath signal at −6 dB relative to the direct signal. For both depictions in FIG. 1 and FIG. 2, the theoretical curve has no data noise, perfect code alignment, and no filtering of the received signal. The observed practical curve has data noise and a two-sided filter bandwidth of 20 MHz. It is clear from FIG. 2, that the resulting range measurement derived from a balanced power measurement of the correlation response function will be corrupted by the multipath component of the received signal.
Conventional DSSS CDMA radio and radio navigation systems, such as GPS, use continuously broadcast signals. Therefore, the direct and multipath signals are both continuously present in the samples that are used in the correlation process. With continuous signals there is no provision for separating the multipath component of the broadcast radio navigation signals from the direct component. Various techniques for multipath mitigation in DSSS CDMA radio navigation systems have been proposed. One such prior art system varies the relative location of the correlation response function power measurements (Pseudorandom Noise Ranging Receiver Which Compensates for Multipath Distortion by Dynamically Adjusting the Time Delay Spacing Between Early and Late Correlators, Fenton et al., U.S. Pat. No. 5,390,207, Feb. 14, 1995). However, this technique requires large receiver bandwidths to operate correctly and cannot separate the multipath from the desired direct signal based on the location of the power measurements on correlation response functions as described in FIG. 2. Another common multipath mitigation technique utilizes post correlation signal-to-noise ratios (Axelrad, P., C. J. Comp, and P. F. MacDoran, “SNR Based Multipath Error Correction for GPS Differential Phase,” IEEE Transactions on Aerospace & Electronic Systems, in press) which also suffers from an inability to separate the multipath from the desired direct signal based on the location of the power measurements on correlation response functions as described in FIG. 2. Another method utilizes post correlation equalization weighting of the response function power estimates to refine the optimal time delay measurement, such as implemented in a rake demodulator (Proakis, Digital Communications, Fourth edition, McGraw-Hill, 2001). However, this technique requires complicated receiver circuitry to operate correctly, and also suffers from an inability to separate the multipath from the desired direct signal based on the location of the power measurements on correlation response functions as described in FIG. 2. All of these prior art multipath mitigation techniques rely on minimizing the impact of multipath signals after the signals have already been absorbed into the correlation process. As shown by the depiction in FIG. 2, techniques that rely on post-correlation power responses will be corrupted with continuous multipath. It is clearly not possible to distinguish the multipath-corrupted signal of FIG. 2 from the direct-only signal of FIG. 1 by changing the relative location of the post-correlation power response measurements.
Radio Navigation Systems Utilizing Pulsed Signals
A pulse is defined as a burst of electromagnetic energy that has limited duration. A pulsed radio navigation signal is comprised of periods where a signal is present, and periods where the transmitter is emitting substantially no output power and therefore a signal is absent.
Previous work on pseudolites (ground based transmitters that generate signals similar in structure to GPS satellite signals) used a pulsing scheme with a long time scale relative to the correlation time. For example, a common pulsing scheme for pseudolites is defined by the Radio Technical Commission for Maritime (RTCM) proposal of 1986 (Parkinson et al., Global Positioning System: Theory and Application, Vol II, AIAA Press, 1996). In this pulsing scheme each full code cycle is divided into eleven (11) slots. Designed for the GPS C/A code, this pulsing scheme resulted in transmitting continuously for 93 chips of the 1023 chip sequence and remaining silent during the remaining code duration. During the next 1023 chips cycle, a different 93 chip sequence would be broadcast. The location of the 93 broadcast chips varied with a known pseudorandom pattern. This pulsing scheme was utilized to minimize the impact of the so-called near-far problem. Although this pulsing scheme reduced the impact of the near-far problem, it did nothing to provide multipath mitigation.
Pulse based radio navigation signals such as Ultra Wide Band (UWB) utilize a pulsing scheme to mitigate multipath (Full Duplex Ultrawide-Band Communication System and Method, Fullerton, U.S. Pat. No. 5,687,169, Nov. 11, 1997 or Time-of-flight Radio Location System, McEwan, U.S. Pat. No. 5,661,490, Aug. 26, 1997). UWB systems broadcast short pulses and provide the capability for both communications and ranging. However, as the name implies, UWB systems spread the broadcast energy in the pulse over a large section (or sections) of radio spectrum. At the core of ranging algorithms for UWB is the detection of the leading edge of the transmitted pulse by measuring the received energy in the radio navigation signal's RF component.
Accurately detecting the leading edge of a pulse requires an extremely wide bandwidth. Typical UWB systems utilize 1 GHz of bandwidth. This is very wide compared to DSSS CDMA systems such as GPS that typically only utilize between 2 and 20 MHz. For pulsed signals, the rise time of the leading edge of the pulse is proportional to the bandwidth. For wide bandwidth signals, the rise time of the pulse is short, allowing for precise timing of when the received power rises above a predetermined threshold. Therefore accurate range determination is possible. For bandwidth limited pulsed signals, the rise time is relatively long resulting in a gradual increase in the power observed in the leading edge of the pulse. With the gradual increase in power, accurately determining the exact beginning of the pulse is impractical.
An alternate UWB technology developed by Aetherwire (Spread Spectrum Localizers, Fleming et al., U.S. Pat. No. 6,400,754, Jun. 4, 2002) utilizes Direct Sequence CDMA (DS-CDMA) providing processing gain from the CDMA processing. The CDMA processing described by Fleming provides a navigation receiver with the capability of computing range measurements from the flight time of the DS-CDMA signal in a manner similar to that described above for GPS. This method further removes the constraint of detecting the leading edge of the pulse. However, the fundamentally large bandwidth requirements of UWB make the technology impractical in many ranging applications. Additionally, the pulsing pattern of Fleming's preferred embodiment utilizes a 10 nanosecond chip length and a code sequence of 1024 chips, which results in a total time of reception of about 10 microseconds. This form of pulsing pattern does not allow sufficient time for multipath to dissipate prior to beginning the transmission of the subsequent chip.
Prior art multipath mitigation techniques in CDMA radio navigation rely on continuous correlation of either continuous or pulsed signals, thereby including the multipath component of the signal in the correlation response function. These prior art techniques are intrinsically limited because the desired direct signal and the undesired multipath signals are conjoined during the correlation process, and therefore become difficult to separate within post-correlation processing. Prior art radio navigation systems that utilize the detection of the leading edge of a pulse to determine range require large signal bandwidth to provide accurate range measurements. This is due to the required rapid rise times in the received pulses.
There is clearly a need for a radio navigation system that can mitigate the deleterious effects of multipath on ranging signals but which does not require (a) large tracts of radio spectrum, (b) large receiver bandwidths, or (c) post-correlation power response interpretation. The present invention achieves these desirable goals by digitally separating the desired direct signals from the undesired multipath signals, prior to the correlation process. This is achieved without requiring special antennas or additional bandwidth beyond that typically utilized by DSSS CDMA radio navigation systems.